1. Field Of The Invention
The present invention relates to knee joint prostheses, and more particularly, to an improved implantable knee joint prosthesis including femoral and tibial components with improved articular surface geometries that allow flexion of the knee about an inclined flexion axis with regard to horizontal, the flexion axis being higher toward the middle of the body and lower toward the outside edge of the body.
2. General Background
In the reconstruction of the anatomical knee joint by total or partial replacement with a prosthetic joint, femoral and tibial prosthetic components provide a knee joint prosthesis in which the articulating or contacting surfaces of the components operate to provide a functioning knee joint.
At the present time, most knee prostheses provide for anteroposterior rotation about a horizontal flexion axis in order to allow movement similar to the anatomical knee joint with the tendons and ligaments of the joint imparting stability and the components affording a certain degree of stability.
Human joints are complex systems which serve a number of functions. Perhaps the most important function is to provide a means of moving body parts for locomotive purposes. The motion provided by most human joints is not simple rotation or translation, but a complex combination of movements.
Perhaps the best known joint with complex motion is the knee. The knee is capable of translation and rotation about three orthogonal axes. Motion is controlled by the collective actions of the articular surface geometries and forces applied to the joint.
Joint replacement devices utilized for pain relief and restoration of function seek to restore normal motion. However, in all cases known to the applicants, the motion allowed by the articular surface geometries (which have been simplified to readily facilitate manufacture and to minimize wear of the articulating components) is abnormally simple. As a result, a "kinematic conflict" may develop at the articular surfaces because the motion allowed by the implants may not be compatible with their relative positions as dictated by external forces.
A kinematic conflict may lead to excessive stresses between the articulating components, leading to destruction of one or both surfaces. Destruction of the surfaces will result in high wear rates, the release of wear debris into the joint, and undesirable tissue reactions. The symptoms are likely to be sufficiently severe to require surgical removal and replacement of the implants, with all of the inherent risks and injuries to the patient.
The above-described problem is particularly true of the reconstructed knee joint in movements from full extension to about twenty degrees of flexion during load bearing activities such as walking. During these activities the normal femur externally rotates about five to fifteen degrees (5.degree.-15.degree.) on the surface of the tibia as the knee flexes twenty degrees (20.degree.). All total knee replacements known to the applicants will not allow this motion to occur without the aforementioned kinematic conflict and negative consequences.
Prior art approaches to this problem include utilizing incongruent articular surface geometries (except for line contact). In these designs, congruence of the articular surfaces is limited to one plane (usually the frontal plane) and results in a line contact between the components. The elasticity of one of the articulating members (usually a polymer tibial component) allows the line contact to expand to an area contact if contact forces are sufficiently large. However, the area contact due to elastic deformation is not sufficient to protect the polymer from excessive stresses and the resulting creep and wear phenomena reported in the literature.
Geometries of this type are advantageous in that they allow relative internal/external rotations of the components without large external forces being applied. However, the contact area between the components diminishes further under these circumstances because the congruent profiles are no longer aligned.
Another approach to this problem has been to simply ignore it, by configuring the articular surface geometries for an area contact and restricting congruent motion to one plane (i.e. flexion and extension only). This approach is advantageous in the sense that it maximizes contact area in pure flexion/extension movements, thus should minimize creep and wear of the polymer.
Designs of this type will not allow internal/external rotation during flexion and extension without substantial effort and stresses between the articulating components. As in the former case, excessive stresses lead to wear and creep of the polymer component and early failure of the joint replacement.
There is yet another approach to this problem known as the meniscal bearing prosthesis. Meniscal bearing prostheses feature an area contact at the articular interface because of congruent surfaces (similar to the Tricon knee). To obviate the problem of excessive constraint inherent in these designs, the polymer insert(s) is designed to be mobile on the surface of a metal plate which is attached to bone. The mobility of the insert(s) helps to avoid a kinematic conflict at the articular interface by allowing the bones to adjust their relative positions according to external forces while maintaining an area contact at the articular interface.
Examples of meniscal bearing designs can be found for example in the Oxford knee (European patent No. 327387-A) and New Jersey Knee (U.S. Pat. Nos. 4,309,778, 4,340,978, and 4,470,158). These designs have several disadvantages. First, the polymer inserts have a tendency to dislocate, requiring surgical intervention for replacement. Second, if all ligamentous structures are not preserved and functional, these designs will not provide the stability required for good function and patient confidence. Additionally, the New Jersey design, while featuring mobile inserts, does not allow the desired motion, because internal/external rotations of the inserts are centered on the metal tibial tray, and not lateral to the center. This inconsistency may lead to a kinematic conflict (especially if all ligamentous structures are preserved and functional).
An example of another prior art type of knee joint presently utilized is disclosed e.g. in U.S. Pat. No. 4,298,992, issued on Nov. 10, 1981 for a "Posteriorly Stabilized Total Knee Joint Prosthesis" wherein there is included a femoral component utilizing a pair of laterally spaced apart condylar portions each of which having an external surface convexly curved to match generally the lateral profile of the anatomical femoral condyle.
U.S. Pat. No. 4,298,992 further discloses a tibial component and a platform portion including spaced apart concavities for receiving each of the condylar portions of the femoral component. The post extends from the tibial plateau into the intracondylar recess of the femoral component so that upon full flexion of the joint, the knee joint is stabilized between the tibial post and femoral recess. The U.S. Pat. No. 4,298,992 addresses the prevention of translocation of the knee during flexion.